The present invention relates generally to optical systems and, more particularly, to an electrically tunable, optical phase shifter for use in an optical phased array antenna.
There are currently very pressing needs for rapid, large-angle pointing and scanning of laser beams of all wavelengths from the infrared through the ultraviolet. These needs include strictly military applications such as laser weapons, which require agile, high energy laser pointing, and laser radar, used for target acquisition and surveillance and for tracking and kill assessment. There are also purely commercial needs such as laser light shows and laser printing, which requires rapid, programmable beam steering. In addition, there are also many areas common to both military and commercial interests such as optical computing and image processing, which require rapid scanning of spatial light modulators, and optical data storage requiring rapid optical addressing. In most of these cases, the impediment to effective performance of the optical system is in the area of beam steering.
Presently available technologies are generally not sufficiently advanced to supply the need for rapid, large-angle pointing and scanning of optical beams and, in particular, of large diameter, diffraction limited carbon dioxide (CO.sub.2) laser radar beams. In many systems, optical beam steering is currently performed using rotating optical elements. Such systems typically consist of galvanometer mirrors and a focal telescopes, performance being limited to beam diameters of somewhat less than six inches, a field of view of approximately five degrees in each direction, and a frame time of approximately one second with a few thousand resolution cells and open loop random access time in the order of ten milliseconds. The capability of handling larger beams is required for higher power systems, particularly for many of the military applications for CO.sub.2 laser radar systems. Larger fields of view and larger apertures, on the order of one-half to one meter, are of great interest, and faster scan times are desired for many applications. In short, there exists a pressing need for an optical version of the versatile phased array antennas now widely used for microwave radar systems.
A static deflector for deflecting a polarized infrared beam is disclosed in U.S. Pat. No. 4,639,091, issued Jan. 27, 1987, to J.-P. Huignard et al. The Huignard et al. deflector comprises a layered square plate having as a front layer a window on which stripe electrodes are disposed. Both the window and the stripe electrodes are transparent to an incident infrared beam. A middle layer of the deflector comprises an electro-optical liquid crystal layer. The bottom layer comprises a substrate having a common electrode adjacent the liquid crystal layer. The common electrode is preferably reflective at the beam wavelength, illustratively it is a gold film; alternatively, for a deflector operating by transmission, a transparent rear plate may be used.
Huignard et al. discloses a periodic staircase waveform comprising N voltage steps which are applied to the stripe electrodes, thereby creating local variations of the refractive index in the liquid crystal layer in such a manner as to form a beam diffraction grating of adjustable period.
The Huignard et al. patent discloses a deflector device comprising a plurality of stripe electrodes, but it fails to disclose an effective way of coupling control voltages to a very large number of stripe electrodes, wherein the electrodes are pitched in the order of the wavelength of light of interest, typically 0.2 to 14 micrometers. Applicants believe that, for the dimensions expressed in the Huignard et al. patent for stripe electrode widths and inter-electrode spacings, there does not currently exist a practical and realizable means for attaching independent control voltages to stripe electrodes which are spaced more densely than approximately 10-20 per millimeter. Applicants anticipate the need for a deflector device wherein a multiplicity of stripe electrodes are pitched in the order of 5-10 micrometers, that is, a density of 100-200 stripe electrodes per millimeter, for operation with light having wavelength of ten micrometers. Clearly, an optical phased array antenna used for rapid, phased-angle pointing and scanning of large diameter laser radar beams, as described earlier, would require a large multiplicity of stripe electrodes, and a correspondingly large plurality of means for coupling control voltages individually to those stripe electrodes.
As an example, a one-half meter aperture phase shifter array, operating on light having wavelength of 10 .mu.meters, requires contacts for 100,000 electrodes, or 2,000 electrodes per centimeter. In order to operate the same phase shifter array at 1 .mu.meter wavelength, contacts for one million electrodes would be required, or 20,000 electrodes per centimeter.